MARSIPOBRANCHII. 



355 



it stops short at the pituitary fossa. There is no cranium, and 

 the spinal cord does not expand anteriorly to form a distinct 

 cerebral mass. The brain, however, may be said to be repre- 

 sented, since the anterior portion of the nervous axis gives off 

 nerves to a pair of rudimentary eyes, and another branch to a 

 ciliated pit, believed to represent an olfactory organ. The 

 generative organs (ovaria and testes) are not furnished with 

 any efferent ducts (oviduct or vas deferens). The generative 

 products, therefore, must be admitted into the abdominal 

 cavity, and gain the external medium by the "abdominal 

 pore." 



ORDER II. MARSIPOBRANCHII (= Cyclostomi, Owen ; and 

 Cyclostomata, Miiller). This order includes the Lampreys 

 (Petromyzonidcz) and the Hag -fishes (Myxinidcz), and is de- 

 fined by the following characters : The body is cylindrical, 

 worm-like, and destitute of limbs. The skull is cartilaginous, 

 without cranial bones, and having no lower jaw (mandible). 

 The notochord is persistent, and there are either no vertebral 

 centra, or but the most rudimentary traces of them. The heart 

 consists of one auricle and one ventricle, but the branchial 

 artery is not furnished with a bulbus arteriosus. The gills are 

 sac-like, and are not ciliated. 



The type of piscine organisation displayed in the Marsipo- 

 branchii is of a very low grade, as indicated chiefly by the 

 persistent notochord without vertebral centra, the absence of 

 any traces of limbs, the absence of a mandible, and the struc- 

 ture of the gills. 



Fig. 133. A, Lamprey (Petromyzon), showing the sucking-mouth and the apertures 

 of the gill-sacs. B, Diagram to illustrate the structure of the gills in the Lamprey : 

 a Pharynx ; b Tube leading from the pharynx into one of the gill-sacs ; c One of 

 i the gill-sacs, showing the lining membrane thrown into folds ; d External opening 

 of the gill-sac. (In reality the gill-sacs do not open directly into the pharynx, but 

 into a common respiratory tube, which is omitted for the sake of clearness.) 



Both the Lampreys (fig. 133, A) and the Hag-fishes are 

 vermiform, eel-like fishes, which agree in possessing no paired 

 fins, to represent the limbs, but in having a median fin running 



